Nowadays, rare earth magnets as typified by Nd magnets are widely used in motors, sensors and other parts built in hard disk drives, air conditioners, hybrid cars, and the like.
Typical rare earth elements used in rare earth magnets include cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), terbium (Tb) and dysprosium (Dy). For the separation of these rare earth elements, the ion exchange resin (or solid-liquid extraction) method and the solvent extraction (or liquid-liquid extraction) method are known. The solvent extraction method is often used in the industrial separation and purification of rare earth elements because the method is capable of efficient large-scale treatment due to continuous steps. In the solvent extraction method, a water phase consisting of an aqueous solution containing metal elements to be separated is contacted with an organic phase consisting of an extractant for extracting a metal element of interest and an organic solvent for diluting the extractant. Then the metal element of interest is extracted with the extractant into the organic phase for separation.
Extraction apparatus known in the art for use in extracting rare earth elements by the solvent extraction (or liquid-liquid extraction) method include a multistage continuous extraction system comprising a plurality of mixer-settlers as shown in FIG. 5 (see Patent Documents 1, 2 and 3). Illustrated in FIG. 5 are an extraction section A for extracting a selected rare earth element from an aqueous phase into an organic phase, a scrubber section B for scrubbing the organic phase, and a back extraction section C for back extracting the rare earth element once extracted in the organic phase into an aqueous phase for recovery. Arrows 1 to 9 indicate lines and flows of aqueous phase, organic phase, and reagents into and out of the mixer-settlers.
A rare earth element-containing aqueous phase from line 1, an extractant-containing organic phase from line 2, and an alkaline aqueous solution from line 3 are fed into the mixer-settler of extraction section A, where the steps of mixing aqueous and organic phases, stationary holding and separating them again are repeated in multiple stages, whereby the rare earth element of interest is extracted from the aqueous phase into the organic phase, which is fed to scrubber section B. The aqueous phase is discharged via line 5, and the rare earth element which remains in the aqueous phase without being extracted into the organic phase is recovered from this aqueous phase. The alkaline aqueous solution from line 3 is fed for adjusting the equilibrium acid concentration.
In scrubber section B, an acid aqueous solution is fed from line 4 as the aqueous phase into the mixer-settler, where the steps of mixing the acid aqueous solution (aqueous phase) 4 with the organic phase having the rare earth element extracted therein in extraction section A, stationary holding and separating them again are repeated in multiple stages. The organic phase 2 is scrubbed or washed in that only the rare earth element, which is contained in the organic phase 2 and should essentially remain in the aqueous phase in extraction section A, is selectively extracted into the acid aqueous solution (aqueous phase) 4, after which the organic phase 2 is delivered to back-extraction section C. On the other hand, the acid aqueous solution (aqueous phase) having only the rare earth element, which should essentially remain in the aqueous phase in extraction section A, selectively extracted therein is discharged via line 9, and fed back to extraction section A. Notably, the acid aqueous solution 4 is adjusted to such an acid concentration that only the rare earth element, which is dissolved in a minor amount in the organic phase 2 and should remain in the aqueous phase, may be selectively extracted.
In back extraction section C, the acid aqueous solution which is adjusted to a sufficient concentration to extract the rare earth element of interest is fed from line 6 as the aqueous phase to the mixer-settler where the steps of mixing the acid aqueous solution (or aqueous phase) 6 with the organic phase 2 scrubbed in scrubber section B, stationary holding and separating them again are repeated in multiple stages, whereby the rare earth element of interest contained in the organic phase 2 is back extracted into the acid aqueous solution (aqueous phase) 6, which is discharged via line 7. The rare earth element of interest is recovered from this acid aqueous solution (aqueous phase) 7. On the other hand, the organic phase 2 from which the rare earth element has been removed by back-extraction is discharged from back-extraction section C and fed back via line 8 to extraction section A for circulation.
In the prior art, a mixer-settler is used to construct each of extraction section A, scrubber section B, and back-extraction section C of the multistage continuous extraction system. For example, a mixer-settler of an arrangement as shown in FIG. 6 is commonly used.
As shown in FIG. 6, the mixer-settler comprises a plurality of liquid-liquid extraction units k connected in fluid communication, each unit having one mixer chamber f with a propeller e mounted therein and four settler chambers g to j connected in series. In the embodiment of FIG. 6, four liquid-liquid extraction units k are connected in four stages. With this mixer-settler, extraction operation is carried out as follows. The flows of aqueous phase are depicted by solid line arrows and the flows of organic phase depicted by broken line arrows. The aqueous phase and organic phase flow in mixer chamber f where they are stirred and mixed by rotating propeller e, then flow in settler chamber g where they dwell for a certain time, that is, the mixture is kept substantially stationary and gradually separates into aqueous and organic phases again. These aqueous and organic phases successively and moderately transfer from settler chamber g to j while separation between aqueous and organic phases proceeds in progress. In the last settler j, the organic and aqueous phases are separated and discharged whereupon they flow into mixer chambers f of liquid-liquid extraction units k of the subsequent stages. Similar operation is repeated in plural stages (four stages in FIG. 6). As shown by solid line arrows (aqueous phase) and broken line arrows (organic phase) in FIG. 6, the aqueous phase and organic phase flow in counter-current, thereby enhancing the transfer rate of extract between aqueous and organic phases, and achieving a high extraction rate.
The multistage continuous extraction system using such mixer-settlers achieves a very high separation efficiency in excess of 99%, enabling separation and recovery of rare earth elements at a very high efficiency. In an example where praseodymium (Pr) and neodymium (Nd) are separated and recovered using mono-2-ethylhexyl 2-ethylhexylphosphate (PC-88A by Daihachi Chemical Industry Co., Ltd.), the system should include extraction section A of 32 stages, scrubber section B of 32 stages, and back extraction section C of 8 stages, summing to 72 stages in total. That is, liquid-liquid extraction units k each having five chambers, one mixer chamber f and four settler chambers g to j are connected in 72 stages in total to construct the multistage continuous extraction system.
As a consequence, the multistage continuous extraction system for separating and extracting rare earth elements becomes a very large scale installation, requiring a very large footprint. A very large volume of liquid is necessary to fill all the chambers of the system therewith. Accordingly, a size reduction of the system would become a great contribution to cost reduction. It is desirable to reduce the size of the system without any loss of separation efficiency.